The present invention relates to a printer, copier, facsimile apparatus or similar electrophotographic image forming apparatus and, more particularly, to a developing device incorporated in such an image forming apparatus for developing a latent image electrostatically formed on an image carrier by a nonmagnetic one-component developer, i.e., toner. The one-component developer or toner is distinguished from a two-component developer which is a mixture of toner and carrier and will be simply referred to as toner hereinafter.
It has been customary with a developing device for the above application to supply toner from a toner supply member to a developing roller or similar toner carrier, form a thin toner layer on the toner carrier by a toner layer forming member, which is pressed against the toner carrier, while charging the toner by friction, and bringing the toner layer into contact with the surface of an image carrier implemented as a roller or a belt. With such a procedure, the developing device develops a latent image electrostatically formed on the image carrier so as to produce a corresponding toner image. Usually, the toner layer forming member is made of metal or silicone rubber or similar elastic material.
The prerequisite with the conventional device using the toner layer forming member is that the pressure acting between the toner carrier and the toner layer forming member be high enough to charge the toner uniformly and form a thin uniform toner layer. This brings about a problem that the toner is apt to adhere to the surface of the toner layer forming member. The amount of toner deposition on the toner carrier locally decreases in portions corresponding to the portions of the toner layer forming member to which the toner has adhered. As a result, the developed image suffers from an irregular density distribution and white stripes and other defects. Moreover, the adhesion of the toner obstructs stable charging and reduces the amount of charge. In addition, the toner is apt to fly about at the position where the toner carrier and toner layer forming member contact each other when the toner carrier is rotated, contaminating the inside of the device.